Recent advances in the development of semiconductor light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs) have made these devices suitable for use in general illumination applications, including architectural, entertainment, and roadway lighting, for example. As such, these devices are becoming increasingly competitive with light sources such as incandescent, fluorescent, and high-intensity discharge lamps.
Light-emitting diodes are current driven devices, meaning that the amount of current passing through an LED controls its brightness. In order to avoid variations in brightness between adjacent devices, the current flowing through the LEDs and their control circuits should be closely matched. Manufacturers have implemented several solutions to address the need to closely control the amount of current flowing through the LEDs. One solution is to keep a constant current flowing through the LEDs using a linear constant current circuit. A problem with using a linear constant current circuit, however, is that the control circuit dissipates a large amount of power, and consequently requires large power devices and heat sinks. In addition, when any non-switched constant current system is dimmed, 0 to 100% dimming is typically not achievable. For example, at lower current levels some LEDs will remain ON whereas others, with higher forward voltages will not.
A more power efficient solution has been attempted which uses a buck-boost regulator to generate a regulated common voltage supply for the high side of the LED arrays. Low side ballast resistors are then used to set the LED current, and separate resistors are used to monitor the current. For example, U.S. Pat. No. 6,362,578 provides a method wherein a voltage converter with feedback is used to maintain a constant load voltage across a series of strings of LEDs and biasing resistors are used for current control. A transistor is connected on the low side of the LEDs and is switched with Pulse Width Modulation (PWM) for brightness control. This design does provide full dimming control as the current is switched, wherein the same current can be maintained when the PWM switch is ON, while not allowing current when the switch is OFF. The average current is then equal to the duty cycle multiplied by the ON current level. The problem with these types of designs is that they are inefficient due to the power losses in the biasing resistor, and may require custom resistors to accurately control the current.
U.S. Pat. No. 4,001,667 also discloses a closed loop circuit that provides constant current pulses, however, this circuit does not allow for full duty cycle control over the LEDs.
U.S. Pat. No. 6,586,890 discloses a method that uses current feedback to adjust power to LEDs with a low frequency PWM signal supplied to the power supply in order to reduce the brightness of the LEDs when in a dim mode. The problem with this method is that if the low frequency signal is within the range of 20 Hz to 20,000 Hz, as disclosed, the power supply can produce audible noise. Also, switching frequencies in this range can thermally cycle the LED's thus likely reducing the reliability and lifetime of the device.
U.S. Pat. No. 6,734,639 B2 discloses a method for controlling overshoots of a switched driving circuit for LED arrays by means of a voltage converter combined with a customized sample and hold circuit. The switching signal controlling the LEDs is linked to a signal to enable and disable the voltage converter and thus it is switching both the load and the supply. The signal controlling the switching of the load is biased such that it operates the switch essentially in its linear region in order to provide peak current control which can result in power losses within the switch, thereby reducing the overall system efficiency. In addition, this configuration is defined as being applicable for frequencies in the range of 400 Hz and does not allow for high frequency switching of the load for example at frequencies above the 20 kHz which is approximately the audible threshold range.
US Patent Application No. 2004/0036418 further discloses a method of driving several strings of LEDs in which a converter is used to vary the current through the LEDs. A current switch is implemented to provide feedback. This method is similar to using a standard buck converter and can provide an efficient way for controlling the current through the LEDs. A problem arises, however when multiple LED strings require different forward voltages. In this scenario, high-side transistor switches are used as variable resistors to limit the current to the appropriate LED string. These high side transistor switches can induce large losses and decrease the overall efficiency of the circuit. In addition, this circuit does not allow a full range of dimming to be obtained.
Therefore, there is a need for a switched constant current driver circuit that efficiently provides voltages to multiple electronic devices according to the forward bias required thereby without the use of biasing resistors or transistors. In addition, there is a need for efficiently dimming light-emitting elements while maintaining a switched constant current.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.